Inhibition of hCG-induced spawning by alpha-methylparatyrosine, a tyrosine hydroxylase inhibitor, in the catfish Heteropneustes fossilis (Bloch).

In this study, the effect of pharmacological inhibition of catecholaminergic activity on hCG-induced spawning was evaluated and correlated with tyrosine hydroxylase (TH) activity, the rate-limiting enzyme in catecholamine biosynthesis. Gravid female H. fossilis collected in both prespawning and spawning phases were given alpha-methylparatyrosine (alpha-MPT: 250 microg/g body weight, ip, an irreversible inhibitor of TH) and human chorionic gonadotropin (hCG: 100 IU/fish, ip) alone or in combination. The fish were sampled at different intervals for measuring hypothalamic and ovarian TH activity and checking spawning response. The administration of hCG resulted in ovulation and spawning in both phases with a higher response in the spawning phase. The administration of alpha-MPT did not induce any response, like the control fish. In the hCG + alpha-MPT groups, the spawning response of hCG was significantly inhibited and delayed by the inhibitor. The spawning response of hCG was accompanied by a significant increase in both hypothalamic and ovarian TH activity at 6 and 12 h of the injection. However, at 24hr the activity decreased except in the spawning phase. The alpha-MPT treatment inhibited TH activity significantly in a duration-dependent manner. In the hCG + alpha-MPT groups, enzyme activity was inhibited at all duration. The results indicate the involvement of catecholamines during the hCG-induced spawning and the specific functional nature of the involvement needs further investigation.